The topologies of a boost converter and a SEPIC (Single Ended Primary Inductance Converter) are generally known in pulsed power supplies. Particularly in the case of two-stage operating devices for discharge lamps, these converter types are widespread in the first stage. The first stage effects a power factor correction and provides an intermediate circuit voltage while, as a rule, a second stage generates a high frequency AC voltage for feeding the discharge lamp. The pulse in the two pulsed power supplies is effected by electronic switches that switch at high frequency compared with a line frequency.
WO 02/41480 discusses the respective advantages and disadvantages of the two converter types that are used in the first stage for the purpose of power factor correction. The essential advantage of the boost converter consists accordingly in a high efficiency, while bounding the lower limit of the output voltage to the peak value of the input voltage is a disadvantage. The characteristics of the SEPIC are the opposite: its output voltage may advantageously be selected independently of the input voltage, while its efficiency is substantially lower than in the case of the boost converter.
WO 02/41480 describes a voltage converter whose topology can be switched over. Depending on the position of a changeover switch, the disclosed voltage converter operates either as a boost converter in a boost mode, or as a SEPIC in a SEPIC mode. The voltage converter disclosed in WO 02/41480 has the following disadvantages: a changeover switch with three poles is required to switch over between the topologies. Although this can be implemented using mechanical switches, an implementation with semiconductor switches is complicated, since two switches are required. In addition, these two switches need to be synchronized. A further disadvantage lies in the fact that an inductor that is required for the SEPIC is switched off in the boost mode. The redundant SEPIC inductor has no function in the boost mode. This has the result that the inductor, which is active both in the SEPIC mode and in the boost mode, is subjected to different loads in the two modes with the same power output at the output of the voltage converter. This inductor needs to be dimensioned such that the maximum amount of energy that can be stored by this inductor in the boost mode is not exceeded. In this case of the SEPIC mode, this inductor is then over-dimensioned. This results in the switchable voltage converter being more expensive than the unswitchable converter in whose mode it is presently operating.
The last-mentioned problems are disposed of by a voltage converter that is known from EP 1 710 898 and is illustrated in FIG. 1. A series connection of a first inductor L1 and an electronic switch S1 is connected between an input terminal J1 and a reference potential M, a first node N1 being formed at the tie point. The voltage converter can be fed between the input terminal J1 and the reference potential M by an energy source whose input terminal J1 generates an input voltage Ue. If appropriate, a filter for reducing radio interference or counteracting overvoltage can further be connected there between.
A series connection of a first capacitor C1 and a second inductor L2 is connected in parallel with the electronic switch S1, a second node N2 being formed at the tie point of the first capacitor C1 and second inductor L2. In addition, a ripple current compensation can be implemented by a magnetic coupling of the inductors L1 and L2.
A series connection of a first diode D1 and a mode switch S2 is connected between the first node N1 and an output terminal J2, the first diode D1 being polarized such that it permits a current flow from the first node N1 to the output terminal J2. An output voltage Ua is present between J2 and the reference potential M. In general, it is buffered by a storage capacitor from which a load, in particular a discharge lamp, draws energy. The so-called intermediate circuit voltage is then present across the storage capacitor. An inverter connected downstream of the voltage converter can generate from the intermediate circuit voltage a high frequency AC voltage that serves to operate a discharge lamp.
With a closed mode switch S2, the voltage converter operates as a boost converter. This is advantageous when a higher voltage is required at J2 than is present at J1.
A second diode D2 is connected with its anode to the second node N2, and with its cathode to the output terminal J2. The voltage converter can therefore operate as a SEPIC when S2 is opened. This is advantageous when a lower voltage is required at J2 than is present at J1. In this case, the control of the mode switch S2 can be performed by a control device 12. The effect of the control device 12 is that the mode switch S2 is open if the voltage at the input terminal J1 exceeds a defined voltage limiting value.
During operation of discharge lamps, the mode switch S2 can also be controlled independently of the voltage that is required by a lamp to be operated at the output J2. If a lamp requires a voltage that is high by comparison with the voltage at the input terminal J1, the mode switch S2 is closed and the voltage converter operates in the boost mode. At comparatively low voltages, the mode switch S2 is opened and the voltage converter operates in the SEPIC mode.
FIG. 2 of EP 1 710 898 shows an exemplary embodiment of a control device 12 of a mode switch S2. A disadvantage of the control device presented there is to be seen, in particular, in the necessity in this case of a high voltage switch, denoted there by S22, for transmitting the control signal, since its reference electrode is coupled to the frame potential, its control electrode to the control signal, but its working electrode is coupled to the intermediate circuit voltage. This results in production costs that are undesirably high.